This invention pertains to digital data demodulators and more particularly to a demodulator for FSK-type signals.
Frequency-shift keying (FSK) is a method of transmitting binary (i.e. digital) data by changing the frequency of the signal between a higher and lower frequency f.sub.1 and f.sub.2 respectively in accordance with the binary value of the data. A typical demodulator for such a system is shown in FIG. 1. In such a demodulator the incoming FSK signal is fed simultaneously into a first bandpass filter 10 and a second bandpass filter 12. Bandpass filter 10 is adapted to pass only signals having a frequency of f.sub.1 while bandpass filter 12 passes only signals having the frequency f.sub.2. The outputs of the filters are fed into two envelope detectors 14 and 16 respectively which produce two signals y.sub.1 and y.sub.2. These signals are respectively proportional to the amplitudes of the signals having the frequencies f.sub.1 and f.sub.2. Signals y.sub.1 and y.sub.2 are fed into an adder 18 which produces a signal Z=y.sub.1 -y.sub.2. Signal Z is fed to a decision circuit 20 which produces either a binary "1" or a binary "0" depending on whether Z is greater than or less than zero, i.e. whether the received signal had the frequency f.sub.1 or f.sub.2.
One of the drawbacks of such a demodulator is that its output baud rate, and therefore the baud rate of the whole system is limited to about half the bandwidth of the signal. Therefore in order to increase the baud rate of the system, its bandwidth must be increased.